Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Overhead electrical transmission cable sag questions

  1. May 28, 2013 #1
    These are questions for electrical engineers involved in electrical power transmission by overhead line and knowledgeable about current practices.

    - What catenary sag is normal, under normal conditions of weather and transmitted current?
    [obviously the answer will depend on the distance between towers and the voltage. What I want is specific examples. Perhaps as a percentage of the span between towers?]

    - Does the sag increase significantly under conditions of extreme hot weather?
    [A percentage increase would be helpful. Is it 1%? 10%? 50%?]

    - Does the current carried heat the cables to produce significant additional sag?
    [Again, a percentage increase would be helpful]

    - Is it the additional sag due to heating that limits the current in overhead cables? Or is it, for example, the need to avoid excessive power loss.

    - What is the safety factor normally used in designing overhead cable transmission systems?

    Thank you for any information.
  2. jcsd
  3. May 28, 2013 #2


    User Avatar
    Gold Member

    There are charts and even computer programs that figure this stuff out. There is definitely a formulated sheet available that would let you do it all by hand. It's definitely a challenge....but it will calculate for any temperature, wind load and also do a calculation for a half inch of ice around the cable. You will also need to use "curved" charts during your calcs....and you need to know every little detail about the cable you are using....that's the challenge.

    The type of material used is of great importance (aluminum, copper, steel, etc....) The coefficient of elastic coefficiency is obviously a huge factor. "curlew" is a wire I onced used for a couple mile overhead line in a steel mill. Curlew is multiple stands of steel (strength) surrounded by mulitiple strands of aluminum (conductors)


    The power company is more concerned with the heat in the wires for voltage drop or power loss, which affects their wallet.....although a factor, they are less concerned with the sag due to heat generation in wire.

    Anchoring these overhead lines at certain "pin points" into the ground will definitely test your statics and dynamics knowledge as well.
    Last edited: May 28, 2013
  4. May 28, 2013 #3
    psparky - thank you. I never doubted that engineers designing cable systems have all necessary design tools at their disposal but having that confirmed does not really help me.

    Could you please give some off-the-cuff ball-park figures, based on your experience, such as I asked for? I want to understand what happens in practice in general terms, so that I have a feel for the magnitudes of sag under different conditions of weather and electrical loading.

    I had imagined that the need to minimize power loss would normally set the current limit, rather than heating. Does that always apply, or does temperature rise sometimes set a limit to current - especially in very hot weather?
    Last edited: May 28, 2013
  5. May 28, 2013 #4


    User Avatar
    Gold Member

    I can give you some guesses based off my memory.....it's been a while. I got paid for a couple months to figure out all this stuff and research it. My questions were similiar to yours, thru the math I got there. And my boss had the computer program, he just wanted me to do it long hand because you simply learn more.

    I don't recall figuring the heat of the wire from the amps......I don't even recall putting in the amount of amps. I believe this stuff is already figured into the multiple other questions they ask you. Doesn't mean it doesn't exist, i just dont recall to be honest.

    I believe the wires sag way more in the summer......even when there is a half inch of ice on cable it does not sag as much as in summer. I believe over a 100 foot span, 5 feet of sag might be a reasonable guess. Keep in mind that the different line phases can not hit so it also depends on how close your wires are side by side as well as vertical.

    I don't know what the safety factor is. Sorry I don't know all the answers, If you really want to know, you need to do the research yourself. Doing the actual calculations will be the greatest teacher of all.

    Like they say, if you want something as bad as you want to breath, you will surely attain it.

    But to maybe answer your question...if a 1" diameter line were hanging at 32 degrees F,with a 3 feet sag, with a 100 feet between poles, the new sag at 100 degrees F would be likely 6 feet.(depending greatly on the type of metal used) If I had the charts in front of me I could give you exact!
    Last edited: May 28, 2013
  6. May 28, 2013 #5


    User Avatar
    Science Advisor
    Gold Member

    And due to the high construction cost to guarantee sag won't be a problem, the trade-offs are considered. Hence sometimes they do make contact with earth causing an outage.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook